A method for quantitative downhole surveillance of a multi-phase flow, comprising at least two phases of either oil, water or gas, in a petroleum well. The method comprising providing at least one shunt chamber (100) which comprises a flow phase separation section (130), a delay chamber (140), one or more outlet ports (122), at least one flow restrictor (126a, 128b) and a tracer release system (124) with one or more tracers. The method comprises separating a shunt flow in the shunt chamber into a low-density flow phase and a high-density flow phase; releasing at least one tracer into the tracer delay chamber, passing the separated flow phases through each flow restrictor and flushing out the phase flows with tracer into the local flow inside the well. The method comprises monitoring the tracers in the production flow and measuring the concentration of the one or more tracers.